Cell/plasma fractioning of blood is traditionally performed by centrifuging, in systems of macroscopic dimensions. More recently, microfluidic techniques have also been developed.
In the field of Microsystems, the technique in the most widespread use is filtering. Filters are placed perpendicularly to the flow with pores of dimensions that are optimized for retaining the particles, thereby enabling a fraction of the liquid phase to be recovered. The main limitation of that technique when applied to a biological solution lies in the great deformability of certain cells (in particular red corpuscles in blood). The pores clog quickly, particularly when the solution is highly concentrated, and the cells end up by lysing.
Another technique consists in performing separation by centrifuging at microfluidic scale, by injecting the suspension into a duct in the form of a spiral or a bend. For example, the article by C. Blattert, R. Jurischka, I. Tahhan, A. Schoth, P. Kerth, and W. Menz entitled “Microfluidic blood plasma separation unit based on microchannel bend structures”, IEEE EMBS, Hawaii 12-15 May 2005, pp. 38-41, describes a device that combines the centrifuging that appears in a channel with a bend and the phenomenon of “plasma skimming” that is due to having a fine channel in the bend. Skimming corresponds to extracting the liquid phase of the blood at a junction, with extraction being the result of the large difference in flow rate between the main channel and the fine channel. That device makes it possible to extract 5% to 10% of the plasma. An improvement of the device, making it possible to achieve a yield of 15% for a hematocrit of 9%, is described in the article by C. Blattert, R. Jurischka, I. Tahhan, A. Schoth, and H. Reinecke entitled “Improved plasma yield by a new deign of microchannel bend structures” μTAS, Tokyo 5-9 Nov. 2006, pp. 359-361 and in document US 2006/020400.
The effectiveness of centrifuging techniques in Microsystems is nevertheless limited by secondary flows (Dean cells) that develop under such conditions and that tend to mix the particles that are to be separated from the liquid fraction. In this context, reference can be made to the article by S. Ookawara, D. Street, and K. Ogawa entitled “Numerical study on development of particle concentration profiles in a curved microchannel”, Chem. Engineering Science 61 (2006, pp. 3714-3724, and the article by A. P. Sudarsan and V. M. Ugaz entitled “Multivortex micromixing” in PNAS (2006), 103, 19, pp. 7228-7233.
In the article by S. Ookawara, R. Higashi, D. Street, and K. Ogawa, entitled “Feasibility study on concentration of slurry and classification of contained particles by microchannel”, Chem. Eng. Journal (2004), 101: pp. 171-178, proposals are made to take advantage of the equilibrium between centrifugal force and the mixing effect induced by the secondary flows in order to extract particles from a suspension.
An emerging technique lies in depleted zone extraction. This technique is based on the fact that particles in suspension injected into a straight duct are subjected to non-uniform lateral migration due to shear forces: this causes a particle-free zone to appear at the edge of the channel, followed by a superconcentrated ring, in turn surrounding a central zone in which concentration is uniform. An application of this technique to extracting blood plasma is described in the article by M. Faivre, M. Abkarian, K. Bickraj, and H. Stone entitled “Geometrical focusing of cells in a microfluidic device: an approach to separate blood plasma”, Biorheology (2006), 43: pp. 147-159. For a sample diluted to a hematocrit of 16% and injected at 200 microliters per hour (μL/h), 24% of plasma is extracted.
The article by J. Park, K. Cho, C. Chung, D. C. Han, and J. K. Chang entitled “Continuous plasma separation from whole blood using microchannel geometry”, IEEE EMBS, Hawaii 12-15 May (2005), pp. 8-9, describes a microfluidic device in which the depleted zone is enlarged by exploiting the high curvature zone (corner) of a bend in a microchannel. That device makes it possible to collect 99% of the cells from a volume of 20 microliters (μL) at a flow rate of 50 nanoliters per minute (nL/min).
The principal limitation of the depleted zone extraction technique is that it is based on an unstable phenomenon. Any action exerted on the flow (e.g. to extract the plasma) gives rise to a flow disturbance. In addition, the depleted zone phenomenon depends on flow conditions (liquid viscosity, rheological characteristics of particles).